Three-by-three matrix converters using pairs of anti-parallel thyristors, preferably silicon controlled rectifiers (SCRs) on one diagonal of the matrix are often called softstarters because of their widespread use in improving the starting, accelerating as well as decelerating and stopping of three-phase squirrel-cage induction motors, which would otherwise be subject to deleterious transients during their starting and stopping phases if they were connected directly to the mains.
Because of the nature of the thyristors, as reflected by their constitutive equations, and the topological structure of this conversion circuit, the evolution of its electrical quantities can evolve in time in highly complex ways, also because the use of SCRs and possibly diodes considerably limits the possibilities of controlling such a circuit.
Despite their intrinsic limitations, the thyristors-based softstarters are widely used to accelerate and decelerate induction motors when no sophisticated quality of motion is required. Their widespread use, still today, is facilitated by their ruggedness and low cost which are still largely unmatched even by the simplest and heaviest-duty motor drives based on voltage-source, two-level, three-phase AC/DC/AC converters.
Additionally, the present availability of low cost, but significantly performing, digital control architectures allows nowadays for employing more sophisticated control techniques for the softstarters which were not economically possible decades ago since the complexity of the required analog/digital circuits rendered them unacceptable in industrial mass production at that time.